1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber preform by an OVD method, and more particularly to a method for producing an optical fiber preform in which the variation of outer diameter in the longitudinal direction is minimized without decreasing the deposition speed and a target quantity of glass is achieved as a whole.
2. Description of the Background Arts
Japanese Patent Application Laid-Open No. 3-228845 discloses a method for manufacturing a large-sized optical fiber preform at high speed by an OVD method. In the disclosed method, a plurality of burners for synthesizing glass particles are arranged in a row at constant intervals, and the row of burners and a starting rod, which are opposed to each other, are reciprocated relatively so as to produce a soot glass deposit body by depositing glass particles around the starting rod which is rotating, and then the soot glass deposit body is vitrified to become a transparent optical fiber preform.
In the method, an attempt is made in the method of reciprocating movement so as to reduce the outer diameter variation of a soot glass deposit body in the longitudinal direction. Every time the starting rod and the row of burners reciprocate relatively, the turning position of the reciprocation is shifted, and when it reaches a predetermined point, the turning position of the reciprocation is shifted in the opposite direction to return to the beginning position. Thus, the turning positions of the reciprocation where the deposition time is long are distributed longitudinally such that the deposited quantity of glass particles become equal in the longitudinal direction, thereby reducing the longitudinal variation of the outer diameter of the soot glass deposit body. In this specification, the number of times of the reciprocation that the turning position of the reciprocation returns to the beginning position is called “the number of equalization-round turns”.
In this method, it is important to determine a schedule for depositing glass particles so that the turning positions may be distributed uniformly over the whole length of the soot glass deposit body. Such schedule for depositing glass particles is established such that the deposited quantity of glass particles is continuously measured with a weight detecting device or the like, and when the measured value becomes close to a target weight, the depositing process may end at the integral multiple of the number of equalization-round turns and the target weight may be attained. Japanese Patent Application Laid-Open No. 3-228845 does not disclose any method for determining such target weight.
Japanese Patent Application Laid-Open No. 10-158025 discloses a method for reducing the outer diameter variation of a soot glass deposit body. In the disclosed method, the outer diameter variation of the whole soot glass deposit body is measured using a central information processing equipment and a CCD camera that can monitor the whole soot glass deposit body such that supplementary deposition of glass particles is made by an auxiliary glass synthesizing burner in the parts where the outer diameter is smaller than that of the other parts. The auxiliary glass synthesizing burner can move the full length of the deposit body independently, thereby reducing the variation in the outer diameter along the whole length.
Also, Japanese Patent Application Laid-Open No. 4-260633 discloses equipment for producing a soot glass deposit body in which the last traverse velocity of burners is determined so as to produce a soot glass deposit body having a target volume by estimating the relevant weight increase based on the data obtained by measuring the weight increase due to each traverse during the depositing process. Japanese Patent Application Laid-Open No. 3-54129 also discloses a method for producing an optical fiber preform in which the traverse velocity or the supply quantity of glass materials in the last traverse is determined in a similar manner. Neither Japanese Patent Application Laid-Open No. 4-260633 nor Japanese Patent Application Laid-Open No. 3-54129 discloses any method for determining a target weight in which glass particles at the tapered portions of the deposit body are taken into consideration.
Japanese Patent Application Laid-Open No. 2-137743 discloses a method for producing an optical fiber preform in which the deposition of soot glass is stopped if the weight per unit length of the soot glass deposit body reaches a predetermined value while measuring the outer diameter and the weight of the deposit body during the depositing process and computing the weight per unit length of the deposit body on the assumption that the length of the deposit body is the summation of the distance of burner movement and the length of the tapered portions of the deposit body. The laid-open patent application does not disclose any method for determining a target weight in which glass particles accumulating at the tapered portions of the deposit body are taken into consideration. Furthermore, no consideration is given to the shape of the tapered portions or the differences in the bulk densities between the effective portion and the tapered portions of the deposit body.